1. Field of the Invention
The present invention relates to an alternator driven by, for example, an internal combustion engine and, more particularly, to a structure of a stator of an automotive alternator mounted on a passenger car, a truck, etc.
2. Description of the Related Art
FIG. 8 is a sectional side elevation of a conventional automotive alternator, FIG. 9 is a perspective view of a rotor of FIG. 8, FIG. 10 is a front view of a stator core applied to a stator of the conventional automotive alternator, and FIG. 11 is a circuit diagram of the conventional automotive alternator. The conventional alternator shown in the drawings includes: a case 3 composed of an aluminum front bracket 1 and an aluminum rear bracket 2; a shaft 5 rotatably disposed in the case 3 and which has a pulley 4 secured to one end thereof; a Lundell-type rotor 6 secured to the shaft 5; fans 7 secured to both ends of the rotor 6; a stator 8 secured to an inner wall surface of the case 3; a slip ring 9 secured to the other end of the shaft 5 and which supplies electric current to the rotor 6; a brush 10 that slides in contact with the slip ring 9; a brush holder 11 accommodating the brush 10; a rectifier 12 electrically connected to the stator 8 to convert alternating current generated in the stator 8 into direct current; a heat sink 13 fitted on the brush holder 11; and a regulator 14 adhesively fastened to the heat sink 13 and which adjusts a magnitude of an alternating voltage generated in the stator 8.
The rotor 6 is equipped with a rotor coil 15 for generating magnetic fluxes by passing an electric current, and a pole core 16 covering the rotor coil 15 in which magnetic poles are produced by the magnetic fluxes. The pole core 16 has a pair of a first pole core assembly 17 and a second pole core assembly 18 that intermesh with each other. The first pole core assembly 17 and the second pole core assembly 18 are made of iron and have claw-shaped magnetic poles 19 and 20 at their ends. Spaces are formed between adjacent claw-shaped magnetic poles 19 and 20 to prevent magnetic fluxes from leaking from between the claw-shaped magnetic poles 19 and 20, and also to function as cooling passages for cooling the rotor coil 15.
The stator 8 is provided with a stator core 22 and two sets of three-phase stator coils 23 in which conductors are wound around the stator core 22 with a phase difference of a 30-degree electrical angle (see FIG. 10 and FIG. 11). The stator core 22 is formed by punching a steel sheet into a comb-like plate with equidistantly arranged teeth, and by rolling or laminating the comb-like plate into an annular shape. An inner periphery of the stator core 22 has slots 25 and teeth 24 that extend in the axial direction.
This example includes the two sets of the three-phase stator coils 23, and the rotor 6 has twelve magnetic poles, two three-phase portions corresponding to each pole. Seventy-two slots 25 and teeth 24, respectively, are formed. The annular stator core 22 has the slots 25 formed at equal intervals of 5-degree mechanical angles (360xc2x0/72). At this time, the seventy-two slots uniformly correspond to the a twelve poles, so that the slots 25 are formed at uniform intervals of 30-degree electrical angles. The two sets of three-phase stator coils 23 in Y-Y connection are respectively provided with a phase difference of a 30-degree electrical angle in the slots 25 and electrically connected to rectifiers 12.
In an automotive alternator having the above construction, current is supplied by a battery (not shown) through the brush 10 and a slip ring 9 to the rotor coil 15 so as to generate magnetic fluxes, whereby claw-shaped magnetic poles 19 of the first pole core assembly 17 are polarized with north-seeking (N) poles, while the claw-shaped magnetic poles 20 of the second pole core assembly 18 are polarized with south-seeking (S) poles.
The pulley 4 is rotated by an engine, and the rotor 6 rotates together with the shaft 5. This causes a rotating magnetic field to be imparted to the stator coil 23, and an electromotive force is generated. The AC electromotive force is converted into direct current by means of the rectifiers 12, and a magnitude of the direct current is adjusted by the regulator 14 before recharging the battery.
In the automotive alternator, one each of slots 25 of the stator core 22 is provided for each set and each phase of the stator coils 23 and for each magnetic pole of the rotor 6. Since there are seventy-two slots 25 and teeth 24, the circumferential widths of the teeth 24 are small; hence, there are fewer chances in which leakage magnetic fluxes produced between adjoining claw-shaped magnetic poles 19 and 20 of the rotor 6 leak out through the same teeth 24.
FIG. 12 illustrates a positional relationship between the teeth 24, the slots 25, and the claw-shaped magnetic poles 19 and 20. FIG. 12A shows a state wherein the teeth 24 overlap with only the claw-shaped magnetic pole 19, FIG. 12B shows a state wherein the teeth 24 overlap with the claw-shaped magnetic poles 19 and 20, and FIG. 12C shows a state wherein the teeth 24 overlap with only the claw-shaped magnetic pole 20. Thus, in the conventional automotive alternator, the teeth 24 have a narrow circumferential width, and the time during which magnetic fluxes leak to the teeth 24 is short. This means a less reduction in effective magnetic fluxes for the stator coils 23 caused by leakage magnetic fluxes, leading to reduced pulsation in magnetic fluxes. An art similar to that of the automotive alternator described above has been disclosed in Japanese Unexamined Patent Application Publication No. 4-26345.
In the conventional automotive alternator having the construction set forth above, if slot openings are arranged at equal intervals of 30-degree electrical angles, then spatial fifth and seventh higher harmonics of magnetic flux density waveforms do not appear. This is illustrated in FIG. 13 prepared by the inventors of the application concerned who have performed analyses of electromagnetic fields. Referring to FIG. 13, the axis of abscissa indicates the interval of slot openings. Electrical angles are at equal intervals at 30 degrees, while they are at nonuniform intervals at, for example, 24 degrees (24 degrees and 36 degrees are repeated). The axis of ordinates indicates a ratio of stator magnetomotive force higher harmonics to a fundamental wave. However, the magnetomotive force higher harmonics of the stator 8 have large spatial eleventh and thirteenth higher harmonics. Hence, if the magnetomotive force higher harmonics of the rotor 6 include the eleventh or thirteenth higher harmonics, then their mutual interference prevents adequate suppression of magnetic flux pulsation, so that fluctuation in a generated voltage cannot be adequately controlled. Thus, there has been a problem in that a magnetic attraction force is produced between the claw-shaped magnetic poles 19 and 20 of the rotor 6 and the stator 8, and resonance takes place in the stator 8, the case 3, etc. or the claw-shaped magnetic poles 19 and 20 of the rotor 6 vibrate, generating noises uncomfortable to an occupant.
Furthermore, if slots are disposed at equal pitches, Ad then marked synchronous pulsation based on the number of slots takes place, producing noises that are even more uncomfortable.
When the gap between adjoining claw-shaped magnetic poles 19 and 20 can be reduced, if the teeth 24 are made narrower accordingly, then a problem arises in that a main magnetic flux decreases although invalid magnetic fluxes can be reduced.
The present invention has been made with a view toward solving the problems described above, and it is an object of the present invention to provide an automotive alternator capable of reducing invalid magnetic fluxes and also reducing uncomfortable noises.
According to one aspect of the present invention, there is provided an automotive alternator comprising: a stator having a stator core in which a plurality of slots that open on an inner peripheral side and extend in an axial direction, and two or more sets of stator coils incorporated in the slots; and a rotor having a rotor coil rotatably provided inside the stator to generate magnetic fluxes by passing an electric current and a pole core which covers the coil and in which a plurality of claw-shaped magnetic poles are formed by the magnetic fluxes, the number of the slots per set, per phase, and per pole being one, the stator coils being disposed such that two sets of three-phase coils have a phase difference from each other, and center-to-center intervals of circumferential gaps of adjoining slot openings being nonuniform, wherein the claw-shaped magnetic poles of the rotor are formed such that at least a narrow tooth of teeth formed between the slots of the opposing stator is positioned to overlap two of the claw-shaped magnetic poles that are adjacent in a direction of rotation of the rotor and have opposite polarities.
In a preferred form of the present invention, the center-to-center intervals of the circumferential gaps of the adjoining slot openings are repeatedly set at electrical angles of xcex1 degrees and (60-xcex1) degrees, the xcex1 degrees ranging from 16 degrees to 29 degrees.
In another preferred form of the present invention, the center-to-center intervals of the circumferential gaps of the adjoining slot openings are repeatedly set at electrical angles of xcex1 degrees and (60-xcex1) degrees, the xcex1 degrees ranging from 22 degrees to 24 degrees.
In yet another preferred form of the present invention, each of the teeth that define the slots of the stator core has a jaw extending from a distal end portion thereof on at least one end in a circumferential direction, and the center-to-center intervals of the circumferential gaps of adjoining slot openings are set to be nonuniform based on an extending length of the jaw.
In still another preferred form of the present invention, the center-to-center intervals of circumferential gaps of the adjoining slot openings are set to be nonuniform based on circumferential widths of the teeth that define the slots of the stator core.
In a further preferred form of the present invention, the stator coils are three-phase stator coils of star connections, and neutral points of the star connections are electrically connected to the rectifiers for rectifying AC outputs.